Lithium batteries, in particular lithium secondary batteries, having such characteristics as a large energy density and a long life span, are used widely as power sources for home appliances such as video cameras and portable electronic devices such as notebook personal computers and cellular phones; recently, applications into large batteries that equip an electric vehicle (EV), a hybrid electric vehicle (HEV) and the like, are anticipated.
A lithium secondary battery is a secondary battery having a structure in which, during charging, lithium melts out from the positive electrode as an ion and moves towards the negative electrode to be stored and conversely, during discharging, the lithium ion returns from the negative electrode to the positive electrode, and it is known that the high energy density of the battery has its source in the electric potential of the positive electrode material.
In addition to lithium transition metal oxides such as LiCoO2, LiNiO2 and LiMnO2 having a layer structure, spinel type lithium transition metal oxide (LMO) of the manganese series such as LiMnO4 and LiNi0.5Mn0.5O4 are known as positive electrode active materials for lithium secondary batteries.
Owing to low raw material costs and the absence of toxicity, which renders it safe, there is a focus on the spinel type lithium transition metal oxide (LMO) of the manganese series as a positive electrode active material for a large battery for an electric vehicle (EV), a hybrid electric vehicle (HEV) and the like. In addition, while excellent power performance characteristics are particularly demanded of a battery for an EV or HEV, on this point, compared to a lithium transition metal oxide such as LiCoO2, which has a layer structure, a spinel type lithium transition metal oxide (LMO), which allows for insertion and desorption of Li ions three-dimensionally, has excellent power performance characteristics. However, with higher efficiency of hybrid electric vehicles (HEVs), currently, further improvement of the power performance characteristics are demanded of the positive electrode active material for HEV batteries.
As a spinel type lithium transition metal oxide (LMO) with improved power performance characteristics, in conventional art, a lithium manganese composite oxide represented by the composition formula Li1+x, Mn2−xOu−yFy (where 0.02≦x, 0.1≦y≦u, 3≦(2u−y−1−x)/(2−x)≦4 and 3.9≦u≦4.1) having a mean particle diameter in the range of 1 to 20 μm is described in Patent Reference 1.
In addition, an Li—Mn series spinel compound represented by the composition formula Li1+xMn2−x−yMgyO4 (x=0.03 to 0.15, y=0.005 to 0.05) having a specific surface area of 0.5 to 0.8 m2/g and a sodium content of 1000 ppm or less is described in Patent Reference 2.
Meanwhile, when cycles are repeated in a high temperature region (for instance, 45 to 60° C.) with a conventional spinel type lithium transition metal oxide (LMO), Mn2+ becomes more prone to elution and the eluted Mn2+ deposits on the negative electrode, which becomes a resistance and causes deterioration of the capacity; thus, it has been told that when putting a spinel type lithium transition metal oxide (LMO) into practical application, the issue lies in the cycle life characteristics in the high temperature region (for instance 45 to 60° C.).
Consequently, in conventional art, as means to increase the cycle life characteristics of spinel type lithium transition metal oxides (LMOs), a method has been proposed, for instance in Patent Reference 3 or the like, whereby a portion of Mn within an LMO is substituted with another element such as Al, thereby stabilizing the spinel structure, suppressing the elution of Mn and suppressing the deterioration of the LMO.    [Patent Reference 1] Japanese Patent Application Laid-open No. H11-045710    [Patent Reference 2] Japanese Patent Application Laid-open No. 2002-033101    [Patent Reference 3] Japanese Patent Application Laid-open No. 2004-186149